1. Field of the Invention
The present invention relates to fault sensors for use in power distribution systems, particularly to fault sensors having telecommunications capability.
2. State of the Art
In the field of power transmission and distribution, generating systems produce electrical power which is transmitted through a grid of electrical high voltage alternating-current (AC), three-phase power lines. Occasionally, a transmission or distribution power line experiences a fault in which, for example, a short circuit or equipment failure on a power line causes a circuit breaker to trip open, causing a power interruption to the customer. Other faults, in particular high impedance faults, can occur when a power line falls onto a high-impedance surface, such as dry grass or an asphalt road, but the wire remains energized because the high impedance surface insulates the down wire to prevent it from generating enough short circuit current to trip the circuit breaker. Or, a down wire may be backfeeding from a service transformer. The backfeeding current will never be high enough to trip the substation protective relay. Another type of voltage problem occurs in a typical three-phase three-wire distribution system where service transformers are delta connected, when there is an open circuit on one of the power lines causing extremely low voltage to the customers located beyond the open circuit.
Various fault sensors have been proposed to detect and report power line faults. One such fault sensor is disclosed in U.S. Pat. No. 5,550,476 of the present assignee, incorporated herein by reference. The fault sensor described in the foregoing patent is microprocessor-based and is provided with a detection algorithm that allows the fault sensor to intelligently distinguish between various different kinds of faults, including momentary outage, sustained outage, overload, inrush, an open line on one to three phases of the circuit with the possibility of a live line on the ground, and voltage sag.
The foregoing fault sensor functions well according to its intended purpose in a homogeneous power distribution system. Presently, however, the power industry is entering an era of deregulation comparable to the deregulation experienced by the telecommunications industry in the 1980's. As a result, the power distribution system will increasingly become a heterogenous system as compared to the homogeneous system of today.
More particularly, a conventional distribution circuit usually has only one power source. Although a distribution circuit may be tied to other circuits originating from other substations, switches between the circuits are opened to prevent parallel circuit operation. This type of distribution system is referred to as a radial system. Prior art fault sensors are designed to detect and identify a faulted line section based on the conventional radial distribution system configuration. Presently, utility operators can identify faulted line sections by tracing tripped fault sensors which have detected the flow of short circuit current. Short circuit current is usually an order of magnitude higher than normal load current. This system only works when there is one supply source per circuit, as is illustrated in FIG. 1, which is typically how the distribution lines are configured today. In FIG. 1, fault sensors S-1 and S-2 are tripped by fault current. The fault location is between the last in a series of tripped fault sensors (S2) and the first normal state fault sensor (S3).
As the utility industry is deregulated, small, dispersed generators are expected to be installed on the distribution system to compete with central generation by the power utility. Distribution circuits with generators are substantially different from radial or single-power-source systems. With conventional fault sensors, in the event of a circuit failure, short circuit current contributed from the substation and the distributed generators will cause all the fault sensors that are on the circuit between distribution substations and distributed generators to trip. As a result, utility operators will no longer be able to locate the faulted line section by tracing the trip fault sensors if the fault occurs between a substation and a distributed generator. Conventional fault sensors are therefore not suitable for use in a heterogenous, multiple-source distribution system.
Interconnection of generators on the distribution circuit will cause conventional prior-art fault sensors to become obsolete. This situation is illustrated in FIG. 2. As seen in FIG. 2, if the same circuit has a distributed generator interconnected to the circuit then a different situation arises than in FIG. 1. All five fault sensors on the circuit will detect short circuit current and thus will be triggered. In particular, fault sensors S-1 and S-2 are tripped by fault current contributed by the substation, and fault sensors S-3, S-4 and S-5 are tripped by fault current contributed by the generator. As a result. operators cannot identify the faulted line section by tracing the tripped fault sensors.
Conventional fault sensors also suffer from further disadvantages. Retaining sensor integrity over the life of the sensor is key to minimizing operation and maintenance cost. In the prior art, in order to monitor line current and voltage field and transmit information relating to the faults occurring on a distribution line, conventional batteries were used to power a microprocessor and a transmitter. The problem arises with such prior art devices that the conventional batteries become exhausted over a short period of time and require replacement or recharging. To manage such conventional battery maintenance and/or replacement at thousands of remote sensing locations has involved considerable expense and has often resulted in failure to detect faults over significant periods of time. Inspecting battery condition in these prior-art devices to insure reliable and proper operation becomes a major maintenance effort.
Another problem with prior fault sensors is that most fault sensors clear themselves from the fault signal after a predetermined time, e.g., four hours. After the fault signal has been cleared, no information is provided as to whether power has been restored to the line. Consequently, troublemen and linemen do not know if there is still a sustained outage or if power has been restored.
What is needed therefore is a fault sensor which can detect faults on a distribution system with distributed generators. The fault sensor should provide for a power-restore signal and, in addition, a low-battery alarm so that maintenance of the sensor can be done on an as-needed basis in a timely manner without implementing a labor-intensive inspection program. The present invention addresses these needs.